<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Main</title>
    <style>
        #parent{
            position: absolute;
            left: 50%;
            top: 50%;
            transform: translate(-50%,-50%); /* 子盒子自身宽高的50% */
            height: 100%;
        }
        #parent div{
            position: relative;
            margin-top: 10%;
            top: 30%;
        }

        a{
            margin-top: 10%;
            font-size: 30px;
        }

    </style>
</head>
<body>
   <div id="parent">
      <div><a href="level1/1.html">level1.1</a></div>
      <div><a href="level2/2.html">level2.2</a></div>
      <div><a href="level3/3.html">level3.3</a></div>
   </div>
</body>
</html>
<script>

</script>